The objectives of this research are to determine: (1)\why cells of the host immune system are often unable to mount an effective response against tumor cells even when those cells bear recognizable distinct tumor antigens; and (2)\the effect of radiation on the host-tumor-cell interactions. This study focuses on the cellular interactions occurring between various types of host cells and between host and tumor cells in situ within the actual tumor, which we have confirmed to contain many tumor-reactive host immune cells. We are using multicellular tumor spheroids of EMT6/Ro mammary sarcoma cells, a relatively immunogenic BALB/c mouse tumor, and Line 1 cells, a weakly immunogenic BALB/c alveolar carcinoma, to study the ability of host immune cells to recognize tumor antigens, infiltrate tumors, and function within a tumor-like microenvironment. The technologies of centrifugal elutriation and immunofluorescent flow cytometry are being used to separate, identify, and purify the subpopulations of in situ host cells. We have determined that EMT7 tumor cells secrete a soluble factor which is highly suppressive to host immune cells and is probably a mechanism which allows this tumor to evade the immune system. (IP)